Spot welding is generally relied upon to join a stacked assembly of members. In recent years, there has been proposed to join such a stacked assembly by friction stir welding (see, for example, Patent Documents 1 through 3).
FIG. 28 shows a stacked assembly 6 comprising a first workpiece W1 and a second workpiece W2, which are stacked together, and placed on a support jig 3, and a friction stir welding tool 7 having a probe 8 embedded in the stacked assembly 6, in the same manner as with an apparatus shown in FIG. 1 of Patent Document 1. The reference characters A3, M in FIG. 28 designate a stirred region which is stirred by the probe 8 and a boundary line between the first workpiece W1 and the second workpiece W2, respectively.
When the stacked assembly is friction-stir-welded, even if the probe 8 is embedded and the material flows plastically, the material cannot project from the lower end face of the first workpiece W1. Therefore, the material of the second workpiece W2 flows plastically to a position near a peripheral side wall of a rotor 9 of the friction stir welding tool 7, and rises to form an annular burr BL. Therefore, a finishing process is required to cut the burr BL away.
As shown in FIG. 29, since the thickness T3 of a region that is compressed by the embedded probe 8 and the thickness T4 of a region of the workpiece W2 directly above a raised portion of the lower workpiece W1 near the embedded probe 8 are reduced, the bonding strength of the stacked assembly 6 is not sufficient.
Furthermore, because the probe 8 is removed after it has been embedded to plastically flow the material in the friction stir welding process, a hole is left in the region from which the probe 8 is removed, tending to reduce the bonding strength of the stacked assembly 6.
To avoid the difficulty that the hole is left in the region from which the probe 8 is removed, Patent Document 2 discloses a technology for holding jigs in abutment against opposite end faces of a stacked assembly, and providing friction stir welding jigs on the sides of the opposite end faces. Specifically, a probe inserted in a through hole in the jig on one side of the end faces is embedded in the stacked assembly to perform a friction stir welding process. Thereafter, the material which flows into a through hole in the jig on the other side of the end faces when the probe is embedded, is pushed back by a probe provided on the other side of the end faces. The projecting material is thus pushed back until the bottom of the hole produced by the removed probe becomes flat, thereby producing flat surfaces.
According to a technology disclosed in Patent Document 3, a stacked assembly is placed on a flat surface of a jig, and an additional material piece is supplied to a region around a probe that is rotated and embedded in the stacked assembly. The additional material piece is welded together with the stacked assembly, filling the hole produced by the removed probe.
The technology disclosed in Patent Document 2 is disadvantageous in that since the two friction stir welding tools need to be installed in opposite directions, the friction stir welding apparatus is complex in structure and the facility investment is expensive.
The technology disclosed in Patent Document 3 results in an increase in the material cost and hence the friction stir welding cost on account of the need for the additional material piece. In addition, because there is a need for a mechanism for supplying the additional material piece, the friction stir welding apparatus is also complex in structure and the facility investment is also expensive.
As shown in FIG. 30, it may be proposed to insert a placement jig 2 having a recess 1 into an insertion cavity 4 in a support jig 3, thereby providing a friction stir welding jig 5. When a stacked assembly 6 comprising a first workpiece W1 and a second workpiece W2 is placed over the recess 1 in the placement jig 2, and a probe 8 of a friction stir welding tool 7 is embedded in the stacked assembly 6, the material of a region A4 is softened to flow plastically, and the material finally flows into the recess 1. Since the material displaced by the embedded probe 8 flows into the recess 1, a burr BL is prevented from being produced. Because the plastically flowing material projects from the lower end face of the stacked assembly, the thickness of the joined stacked assembly 6 is not reduced, and the desired bonding strength is achieved.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-178168
Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-259863
Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-62678